Process for the conversion of synthesis gas to oxygenates

Description

[0001] The present invention relates to an improved process for the conversion of carbon oxide(s) and hydrogen containing feedstocks to oxygen containing hydrocarbon compounds in the presence of a particulate catalyst.

[0002] In particular, the present invention relates to an improved process for the conversion of carbon oxide(s) (CO and CO2) and hydrogen containing feedstocks, e.g. synthesis gas or syngas, to alcohols in the presence of a particulate modified molybdenum sulphide based catalyst, or a modified methanol based catalyst and/or a modified Fischer-Tropsch catalyst.

[0003] US 4,122,110 relates to a process for manufacturing alcohols, particularly linear saturated primary alcohols, by reacting carbon monoxide with hydrogen at a pressure between 20 and 250 bars and a temperature between 150 DEG and 400 DEG C., in the presence of a catalyst, characterized in that the catalyst contains at least 4 essential elements: (a) copper (b) cobalt (c) at least one element M selected from chromium, iron, vanadium and manganese, and (d) at least one alkali metal.

[0004] US 4,831,060 relates to the production of mixed alcohols from carbon monoxide and hydrogen gases using a catalyst, with optionally a co-catalyst, wherein the catalyst metals are molybdenum, tungsten or rhenium, and the co-catalyst metals are cobalt, nickel or iron.

[0005] The catalyst is promoted with a Fischer-Tropsch promoter like an alkali or alkaline earth series metal or a smaller amount of thorium and is further treated by sulfiding. The composition of the mixed alcohols fraction can be selected by selecting the extent of intimate contact among the catalytic components.

[0006] Journal of Catalysis 114, 90-99 (1988) discloses a mechanism of ethanol formation from synthesis gas over CuO/ZnO/Al2O3. The formation of ethanol from CO and H2 over a CuO/ZnO methanol catalyst is studied in a fixed-bed microreactor by measuring the isotopic distribution of the carbon in the product ethanol when 13C methanol was added to the feed.

[0007] It is an object of the present invention to provide an improved process in term of selectivity and catalyst activity and operating life for the conversion of carbon oxide(s) and hydrogen containing feedstocks to oxygen containing hydrocarbon compounds in the presence of a particulate catalyst.

[0008] In particular, the present invention relates to an improved process in term of selectivity and catalyst activity and operating life for the conversion of carbon oxide(s) and hydrogen containing feedstocks, e.g. synthesis gas or syngas, to alcohols in the presence of a particulate modified molybdenum sulphide based catalyst, or a modified methanol based catalyst and/or a modified Fischer-Tropsch catalyst.

[0009] The present invention thus provides a process for the conversion of carbon oxide(s) and hydrogen containing feedstocks to oxygen containing hydrocarbon compounds in the presence of a particulate catalyst comprising the step of reacting carbon oxide(s) and hydrogen in the presence of a particulate catalyst in a conversion reactor to form oxygen containing hydrocarbon compounds characterised in that an ether is added to the conversion reactor.

[0010] In particular, the present invention provides a process for the conversion of carbon oxide(s) and hydrogen containing feedstocks, e.g. synthesis gas or syngas, to alcohols in the presence of a particulate modified molybdenum sulphide based catalyst, or a modified methanol based catalyst and/or a modified Fischer-Tropsch catalyst comprising the step of reacting carbon monoxide and hydrogen in the presence of said catalyst in a conversion reactor to form alcohols characterised in that an ether is added to the conversion reactor.

[0011] According to a preferred embodiment, the present invention provides a process for the conversion of hydrocarbon to alcohols comprising the steps of:

1. converting hydrocarbon to a mixture of carbon oxide(s) and hydrogen in a syngas reactor,

2. passing the mixture of carbon oxide(s) and hydrogen from the syngas reactor to a conversion reactor, and

3. reacting said mixture in said conversion reactor in the presence of a particulate modified molybdenum sulphide based catalyst, and/or a modified methanol based catalyst and/or a modified Fischer-Tropsch catalyst to form alcohols,

characterised in that an ether is added to the conversion reactor.

[0012] For the purpose of the present invention and appended claims, producing oxygen containing hydrocarbon compounds from mixture of carbon oxide(s) and hydrogen (e.g. synthesis gas) means that the hydrocarbon oxygenates represent at least 10% by weight of the products obtained from the conversion reactor, preferably at least 20% by weight, more preferably at least 40% by weight.

[0013] According to a preferred embodiment of the present invention, the oxygen containing hydrocarbon compounds are alcohols and ethers. The alcohols are mainly methanol, propanol, ethanol and butanols (predominately n- butanol and isobutanol); said methanol, propanols (predominately n-propanol with low amounts of iso-propanol) ethanol and butanol preferably represent together at least 5% by weight of the products obtained from the conversion reactor, more preferably at least 10% by weight, most preferably at least 20% by weight. The ethers may represent all together at least 1% by weight of the products obtained from the conversion reactor, more preferably at least 2% by weight.

[0014] According to another embodiment of the present invention, water and carbon dioxide are also produced in the conversion reactor; then, water, alcohols and ethers preferably represent together at least 80% by weight of the net products obtained from the process.

[0015] According to an embodiment of the present invention, the ether which is added to the conversion reactor comes directly from the oxygenates obtained from the conversion reactor as by-products. Said ether is thus preferably separated from the alcohols produced in the conversion reactor and sent back to the said conversion reactor.

[0016] According to another preferred embodiment of the present invention, the ether which is added to the conversion reactor is a methyl, ethyl, propyl and/or butyl ether, preferably a mixture of at least two of these ethers. Preferably the ether which is added to the conversion reactor is selected from ethanol and propanol derived ether(s) such as diethyl ether, n-propyl ether, ethyl n-propyl ether, ethyl isopropyl ether, n-propyl isopropyl ether and iso-propyl ether, or even preferably a mixture of at least two of these ethers.

[0017] Quite surprisingly, the addition and/or recycle of even tiny amounts of ether to the conversion reactor have proven to be highly beneficial to the alcohols selectivity, especially the ethanol selectivity, while simultaneously increasing catalyst activity and improving operating life. The addition of larger quantities of ethers has been found to confer additional benefits of reduced water and carbon dioxide production.

[0018] Beyond these unexpected advantages, other advantages have also been found when applying the present process invention, amongst others:

(i) less waste, less by-products and thus higher carbon efficiency.

(ii) less capital, fewer separations, reduced storage tanks.

[0019] As indicated, the particulate catalyst used in the conversion reactor is preferably a modified molybdenum sulphide based catalyst, and/or a modified methanol based catalyst and/or a modified Fischer-Tropsch catalyst.

[0020] Preferably, the catalyst used in the present invention contains at least molybdenum and/or copper; it is preferably promoted by the addition of an alkali metal salt. Molybdenum sulphide based catalysts are preferred; a salt of potassium, especially potassium carbonate, is the preferred promoter.

[0021] Most preferably, the catalyst used is a molybdenum sulphide based catalysts containing cobalt, the molybdenum to cobalt molar ratio being preferably comprised between 1.5 and 2.5, more preferably 2; the said molybdenum sulphide based catalysts containing cobalt is most preferably promoted with potassium carbonate.

[0022] According to another embodiment of the present invention, the ether compound added to the conversion reactor does not come from the direct recycling of a compound produced in the said conversion reactor. Preferably, the ether comes from an indirect route, e.g. from the separation from olefins obtained during a subsequent step of converting the alcohols into corresponding olefins, or from a syngas to dimethyl ether process or etherification of methanol.

[0023] According to an embodiment of the present invention, the catalyst used in the conversion reactor can be selected to not produce any ether compound.

[0024] According to another embodiment of the present invention, the ether compound added to the conversion reactor does come from the direct recycling of a compound produced in the said conversion reactor.

[0025] According to an embodiment of the present invention, the catalyst used in the conversion reactor can be selected to produce any ether compound in addition to alcohols.

[0026] The hydrocarbon feedstock used for syngas generation is preferably a carbonaceous material, for example biomass, plastic, naphtha, refinery bottoms, smelter off gas, municipal waste, coal and/or natural gas, coal and natural gas being the preferred ones, most preferably natural gas.

[0027] Processes for producing mixtures of carbon oxide(s) and hydrogen (synthesis gas) are well known. Each has its advantages and disadvantages and the choice of using a particular reforming process is dictated by economic and available feed stream considerations, as well as by the desired mole ratio of H2:C0 in the feedstock resulting from the reforming reaction. The synthesis gas may be prepared using any of the processes known in the art including partial oxidation of hydrocarbons, steam reforming, gas heated reforming, microchannel reforming (as described in, for example, US 6,284,217 which is herein incorporated by reference), plasma reforming, autothermal reforming and any combination thereof. A discussion of these synthesis gas production technologies is provided in "Hydrocarbon Processing" V78, N.4, 87-90, 92-93 (April 1999) and "Petrole et Techniques", N. 415, 86-93 (July-August 1998). It is also envisaged that the synthesis gas may be obtained by catalytic partial oxidation of hydrocarbons in a microstructured reactor as exemplified in "IMRET 3: Proceedings of the Third International Conference on Microreaction Technology", Editor W Ehrfeld, Springer Verlag, 1999, pages 187-196. Alternatively, the synthesis gas may be obtained by short contact time catalytic partial oxidation of hydrocarbonaceous feedstocks as described in EP 0303438. Preferably, the synthesis gas is obtained via a "Compact Reformer" process as described in "Hydrocarbon Engineering", 2000, 5, (5), 67-69; "Hydrocarbon Processing", 79/9, 34 (September 2000); "Today's Refinery", 15/8, 9 (August 2000); WO 99/02254; and WO 200023689.

[0028] Any hydrocarbon-containing feed stream that can be converted into a feedstock comprising carbon monoxide and hydrogen, most preferably a synthesis gas (or "syngas"), is useful in the processes of the invention. The ratio of hydrogen to carbon monoxide in the reaction zone is preferably in the range of 20:1 to 0.1:1 by volume, more preferably in the range of 5:1 to 0.2:1, most preferably in the range of 1.5:1 to 0.5:1, e.g. 1:1. The alcohol synthesis catalysts can also catalyse the water gas shift reaction. A consequence of this is that hydrogen and carbon dioxide are interconvertable with carbon monoxide and water. For high partial pressures of carbon dioxide (at or above the water gas shift equilibrium), carbon dioxide can act as a carbon monoxide source and a hydrogen sink and this can effect the apparent preferred syngas ratio Useful feed streams include natural gas (mainly methane, but natural gas composition can vary depending on location and source), naphtha, refinery off-gas, LPG, gas oil, vacuum residuals, shale oils, asphalts, various types of fuel oils, coal based /lignin deposits and hydrocarbon containing process recycle streams. According to a preferred embodiment of the present invention, methane is used as the hydrocarbon-containing feed stream to be converted into carbon oxides(s) and H2.

[0029] Feedstocks comprising carbon monoxide and hydrogen, e.g., synthesis gas may undergo purification prior to being fed to any reaction zones. Synthesis gas purification may be carried out by processes known in the art. See, for example, Weissermel, K. and Arpe H.-J., Industrial Organic Chemistry, Second, Revised and Extended Edition, 1993, pp. 19-21.

[0030] The particular reaction conditions for the conversion reactor embodiments described below are not narrowly critical and can be any effective reaction conditions sufficient to produce mainly oxygen containing hydrocarbon compounds. The exact reaction conditions will be governed by the best compromise between achieving high catalyst selectivity, activity, lifetime and ease of operability, as well as the intrinsic reactivity of the starting materials in question and the stability of the starting materials and the desired reaction product to the reaction conditions.

[0031] In one embodiment of this invention, feedstock comprising the desired molar ratio of H2:CO is fed to a conversion reactor at a controlled rate and the reaction is carried out in a reaction zone under controlled conditions of temperature and pressure in the presence of a catalyst to convert the feedstock into oxygenates. The temperature in the reaction zone is selected from the range of from about 150°C to about 400°C, preferably a temperature in the range of from about 200°C to about 350°C and most preferably from 250-330°C. The gas hourly space velocity (GHSV) of the feedstock (liters of feedstock/hr/liter of catalyst) passing through the reaction zone can vary significantly, depending upon a variety of factors such as, for example, reaction conditions, composition of the feedstock and quantity and type of catalyst being used. The GHSV can be maintained at any rate in the range of from about 1 to about 30,000 hr-1 or more, preferably will be maintained at a rate of at least about 500 hr-1, and more preferably will be maintained at a rate of at least 1,000 hr-1. The pressure in the conversion reaction zone may be selected from the range of from about 5 to 200 bar, preferably a pressure in the range of from about 50 to 150 bar and most preferably at 80-120 bar. The hydrogen and carbon monoxide partial pressures should be sufficient to enable the production of oxygenates. Hydrogen and carbon monoxide may be fed separately to the conversion reactor or, preferably in combination, e.g., as synthesis gas.

[0032] For purposes of this invention, GHSV is gas hourly space velocity which is the rate of gas flow over the catalyst. It is determined by dividing the volume of gas (at 25°C. and 1 atmosphere) which passes over the catalyst in one hour by the volume of the catalyst. LHSV is liquid hourly space velocity which is the rate that the liquid organic substrate is fed to the conversion reactor. It is determined by dividing the liquid volume pumped in one hour by the volume of catalyst.

[0033] The conversion to oxygenates reaction can be carried out by passing the mixture of hydrogen and carbon monoxide over the conversion catalyst as a vapor phase reaction or as a liquid phase reaction, e.g., slurry reaction or trickle bed fluidized bed reactor.

[0034] The reaction may be carried out in any appropriate reactor, e.g. a tubular reactor using a fixed bed of the catalyst. The reactants may be fed to the catalyst by feeding down or up, or a combination of both, to a fixed bed located in a tubular reactor. It may be desirable to use a reactor design that operates by plug flow and causes minimal turbulence in the reactor zone. The reaction may be effected in a dynamic bed of the catalyst. In such a reaction, the bed of catalyst is moving such as in the case of a fluid bed of the catalyst. The alcohols conversion reactor may preferably be chosen amongst tubular, multitubular, slurry, moving bed, fluidised bed, radial bed, multibed or reactive distillation reactor. It is preferably a multibed or multitubular vapour phase reactor.

Claims

1. Process for the conversion of carbon oxide(s) and hydrogen containing feedstocks to oxygen containing hydrocarbon compounds in the presence of a particulate catalyst comprising the step of reacting carbon oxide(s) and hydrogen in the presence of a particulate catalyst in a conversion reactor to form oxygen containing hydrocarbon compounds characterised in that an ether is added to the conversion reactor.

2. Process according to claim 1 wherein the carbon oxide(s) and hydrogen containing feedstocks are synthesis gas or syngas, the oxygen containing hydrocarbon compounds are alcohols, the particulate catalyst is a particulate modified molybdenum sulphide based catalyst, or a modified methanol based catalyst and/or a modified Fischer-Tropsch catalyst.

3. Process for the conversion of hydrocarbon to alcohols comprising the steps of

a. converting hydrocarbon to a mixture of carbon oxide(s) and hydrogen in a syngas reactor,

b. passing the mixture of carbon oxide(s) and hydrogen from the syngas reactor to a conversion reactor, and

c. reacting said mixture in said conversion reactor in the presence of a particulate modified molybdenum sulphide based catalyst, and/or a modified methanol based catalyst and/or a modified Fischer-Tropsch catalyst to form alcohols,

characterised in that an ether is added to the conversion reactor.

4. Process according to any of the preceding claims wherein the ether which is added to the conversion reactor is selected from methyl, ethyl, propyl and/or butyl ether, preferably a mixture of at least two of these ethers.

5. Process according to claim 4 wherein the ether is selected from ethanol and propanol derived ether(s) such as diethyl ether, n-propyl ether, ethyl n-propyl ether, ethyl isopropyl ether, n-propyl isopropyl ether and iso-propyl ether, or even preferably a mixture of at least two of these ethers..

6. Process according to claim 4 wherein the ether is dimethyl ether.

7. Process according to any of the preceding claims wherein the ether which is added to the conversion reactor comes from the oxygenates obtained from the conversion reactor as direct by-products.

8. Process according to claim 7 wherein said ether is separated from the alcohols.

9. Process according to claims 1 to 6 wherein the ether compound which is added to the conversion reactor comes from an indirect route, e.g. from the separation from olefins obtained during a subsequent step of converting the alcohols into corresponding olefins.